DE2003712C3 - N-path filter using a digital filter as a time-invariant component - Google Patents

Description

30th

The invention relates to a / V path filter formed from a plurality of input modulators for modulating a supplied input signal, one to the Outputs of the modulators connected filter arrangement for the modulated input signal and one A plurality of output demodulators connected to the output of the filter arrangement for demodulation of the filtered signal.

Time-variant networks of the type of an N-path filter now play a well-defined role in Area of network theory. The common use of integrated circuits today is N-path networks particularly advantageous, since with this help bandpass transmission properties can be achieved without using Realize inductive elements.

An N-path system generally has a time-invariant network with 2N ports in series with Input and output modulators on. Typically, each path in the system contains an input modulator, a time-invariant network and an output modulator. The input and output modulation signals for each path are periodic, usually identical signals and differ from route to route by a fixed time delay. An exhaustive explanation of such systems can be found in an essay "An alternative approach to the realization of network transfer functions: the ΛΑ-path-filter" in the journal "Bell System Technical Journal", 9/1960, pages i 321 to 1350.

A major disadvantage of existing N-path filters is the requirement that the transmission properties of each Path are essentially identical, so that time-variant modulation products can be found at the output of the Eliminate the N-path system by interference. Because of 6s This requirement has heretofore been limited to the use of N-path systems.

The invention has accordingly set itself the task of creating N-path filters in which the Requirement for identical transmission properties of each path can be met in a simple manner.

To solve this problem, the invention is based on an N-path filter of the type mentioned at the beginning and is characterized in that the filter arrangement comprises a sampler multiplexer for forming Multiplex sampling pulses of the modulated input signal, a single digital filter circuit for filtering the multiplex sampling pulses and a demultiplexer circuit for demultiplexing the filtered signal from the digital filter circuit

According to the basic concept of the invention is therefore using a uniform Digital filter that is operated with a time sequence that merges applied signals in a time division multiplex process allows, the requirement for a multitude of networks is simplified. There is only one filter is used, the transmission properties do not have to be adapted to one another, because each Signal passes through the same network

The invention is described in more detail below with reference to the drawings. It shows

F i g. 1 a typical N-path filter system,

F i g. 2 the N-path filter system according to the invention using a digital filter,

F i g. 3 shows a modification of the first three stages according to FIG. 2,

Figs. 4A and 4B show several modulation waveforms used in the invention,

Fig. 5 shows a unified modulator-multiplexer circuit according to the invention,

F i g. 6 shows a number of switching waveforms for the circuit of FIG. 5,

7 shows a waveform generating device used in the invention;

F i g. 8 and 9 show a number of waveforms used in the invention;

Fig. 10 logic circuits for controlling the multiplexing operation the circuit according to FIG. 5.

Figure 4 shows a conventional N-path filter of the type described, for example, in the aforementioned reference. A supplied input signal x (t) is limited in its bandwidth and then modulated in N modulators 11-1 to HN. Each modulated signal is then fed to a low pass filter 12-1 to 12-N. The output signal of each filter is then in turn modulated in one of the modulators 13-1 to 13-N, after which all N signals are combined additively in a combination circuit, ie summing circuit 14, to generate an output signal y (t) after further filtering. The modulation signals p \ (t) ... Pn (O and q \ (t) ... q ^ ift) can be a sequence of identical periodic waves that are shifted in their phase position by the factor T / N

where T is the girund period - ^ of the waves p (t) and

q (t) and N corresponds to the number of paths in the system. Since the transfer function of the system shown can be represented by the transfer function of one of the low-pass filters 12 converted in frequency and arranged symmetrically with respect to the center of each of the frequency components of p (t) , the system shows the properties of a band-pass filter. One of the main problems in the practical implementation of the N-path filter shown is the mutual adaptation of the transmission properties of each path. Ideally, each low pass filter should be 12-1 through 12-N

be identical if time-variant modulation products should be canceled out by interference at the output of the system. Furthermore, the properties each modulator and each path may be identical in a corresponding manner. The person skilled in the art recognizes that such Requirements are easy to implement in theory, but not in practice.

An inventive system which overcomes these difficulties, is shown in Figure 2, a signal fed to x (t) is passed through a band limiting filter 15 ztsr removing unwanted signal components. The filtered signal f (t) is then fed to a plurality of modulators 11-1 to UN which in all respects correspond to the functions shown in FIG. 1 shown input modulators are identical. According to the following explanation, a large number of input modulators are not generally required in the practical implementation of the invention.

However, the explanation of the system shown should still be discussed in further developments of the invention make it easier to understand.

The N-modulated signals are applied to a sampler multiplexer 17. This device, which can be of any known type, samples the AZ-modulated signals in a known manner and generates a serial signal sequence of the various sampling pulses. A conventional analog / digital converter 18 encodes the multiplex sampling pulses and passes them to a digital filter 19. There the encoded pulses are processed according to a predetermined filter scheme. Any known type of digital filter can be used. The synchronization between the filter 19 and the sampler multiplexer 17 is ensured by sampling clock signals which are applied from the filter 19 to the device 17 via the line 16. The digital signals emanating from the digital filter 19 are passed to a demultiplexer 21 which, as its name suggests, N parallel output signals which represent the digitally filtered modifications of the original W signals applied to the device 17. The digital-to-analog converter 22 operates sequentially with the aid of a switch 23a and converts the N digitally filtered signals into N analog signals. As the dashed line indicates, the changeover switch 23b is synchronized with the changeover switch 23a and therefore outputs each of the / V signals to a shunt capacitor Ci, Ci ... Cn , which smooths the amplitude of the converted signals. If desired, the capacitors can be replaced by holding networks of the usual type. The modulators 13-1 to 13- / V, which are associated with the correspondingly designated modulators in FIG. 1 are identical, demodulate each of the N signals. These are then combined additively by a network 14. To eliminate interfering components, the resulting signal is limited in its bandwidth by a filter 24, after which the desired output signal! y (t) arises. The signals of each of the A / paths are processed by the same filter instead of a group of similar filters. The only components that are not shared are the capacitors and the input and output modulators. As will be explained later, the input modulators 11-1 to HN can be implemented in a form that avoids duplication of the input modulation device. Accordingly, the problem of precisely matching the transmission properties of a plurality of filters is substantially alleviated, and it has accordingly been found that time-variant modulation products are suppressed by 10 to 20 dB better than in conventional Λί-path filters.

It can be shown that when N is an even number

the / V path system can be implemented using only y-input channel modulators. If, for example, N equals 4, then the / v "-path system can be set up using only two modulators and two modulation signals, where the second modulation signal is converted to the first

Delayed by ~ r seconds ie in the phase by ~ rad

■ * N

deviates Fig. 3 shows, for example, in the form of a block diagram, the first three stages of the system according to Fig.2. As has been explained above, the bandwidth of the input signal x (t) \ n is limited by the filter 15 to generate a signal f (t) which is modulated in the modulator 11 by signals m (t) and with-). The product of f (t) and the first modulation signal m (t) is denoted by M _s and the product of f (t) and m (t - ^ -j is denoted by M ₂ Mi ₅ and Af ₂₅ represent the nested ones by the scanner -Multiplexer 17 represents samples of the signals M _x and M2 generated.

In the ideal case, the modulation signal m (t) should be a pure sine wave with the desired bandpass center frequency. As it is extremely desirable

jo that / V-path filters are tunable, d. That is, in order to easily change the center frequency of the desired passband, a system using a sinusoidal modulation signal would require a tunable sinusoidal oscillator and tunable phase shifters to generate the modulation signals required. Unfortunately, however, the accuracy required for / V path systems cannot be economically achieved using such facilities. An alternative is to use a modulation signal consisting of square-wave pulses with a repetition frequency equal to the desired center frequency. A pulse train of this type is easy to generate, but pulse-shaped signals are so rich in harmonic components that they create difficult operating conditions for the N-path system. A modulation signal which has many advantages of the signals described above and few of their disadvantages is the multistage approximation phase 1 of a sinusoidal signal m (t) shown in FIG. 4A. 4B shows phase 2, namely that

same signal delayed by -j- seconds, that is

According to the basic idea of the invention, it is not necessary that the specified modulation signals be generated in individual modulators with the applied input signal, and finally multiplexed by separate devices. Instead of an operational amplifier circuit structure, for example according to FIG. 5, for simultaneous Generation, mixing and multiplexing of the desired signals can be used. Corresponding 5 becomes that of the band limiting filter 15

ft5 according to FIG. 2 or 3 outgoing signal f (t) is given to two paths containing resistors and transistor switches Q], Q ₂ , etc., which are connected to the operational amplifier 31. At the output of the scanner 41, the

is acted upon by the operational amplifier 31, the desired modulated, sampled multiplex signals Mi. «, M ₂₅ , etc. are produced. If desired, the scanner 41 can be easily incorporated into the operational amplifier circuit. The circuit s according to FIG. 5 has an amplifier characteristic corresponding to the modulation signal shown in FIG. 4A. If the supplied signal f (t) is amplified in accordance with such a characteristic curve, it can be seen that the resulting output signal is identical to a signal which is formed by mixing the input signal with the modulation signal shown. The transistor switches Qi, Q ₂ , Q and Q ₄ are operated in a saturated manner as indicated under the waveforms in FIGS. 4A and 4B. The reinforcement of the device shown can be expressed as follows:

G - gain with Qi on, Q ₂ off, Qz, Q * off;

- G - gain with Qi on , Q \ off, Qi, φι off; OtG - gain with Q \ on , Q ₂ off, Q ₃ , Q _{4 on} ;

- ocG - gain Qi on , Qi off, Qj, Q * on ,

where, in a case chosen as an example, α is equal to 0.414 and G is a preselected gain, for example G = I ₁ OO. If only one condition is considered as an example, for which Qi is switched on and Q ₂ , Qj, Q _{4 are} switched off, then the resistor Ri is earthed and the signal f (t) is applied to the positive terminal of the amplifier 31 via the parallel connection of the resistors R. ₂ ', R ₂ \, R ₂₂ in series with Ra. In this case it can be shown that the gain G of the modulator circuit is equal to Rn divided by the total resistance of the series connection of the resistor Ra with the resistor circuit R ₂ , R ₂ and R22 times the quotient of (Rn + Ri) and Rs . Typical values for the circuit shown in FIG. 5 resistors used are given in the following table:

40

Ri ' = 10,000X :, R ₂ ' = 9,015 Jt, R ₁ = 12,222A-, «4 = 1,759 Jt, Rn = 17.22 Jt, Rn = 4.194 mill. Alles - 6.307 Jt, Ru = 3,693 Jt. R ^ = 5.68 it, Rn = 2.00 Jt.

Flip-flops change their state when the signal applied to their connection "T" becomes "0", so it can be seen that all the desired curve shapes from the circuit according to FIG. 7 can be generated.

What is needed is a multiplex network that is alternately connected to transistors Qi, Q ₂ , etc. in FIG. 5 the in F i g. 6 applied switching waveforms for phases 1 and 2. It is also necessary that a sample of each modulated waveform be generated and that the samples alternate between the two modulation phases at the correct frequency. If the sampling clock signal generated by the digital filter 19 (FIG. 2) is a square wave of the type shown in FIG. 9, whose basic frequency is twice as large as the sampling frequency f _s for one phase, the condition can be specified that the transistor switches according to F i g. 5 applied switching signals change from one phase to the other and that with each leading edge of the curve shape shown, a narrow sample is taken at the modulator output. In summary, it then results overall that two phases of the modulated signal are alternately sampled.

In the circuit according to FIG. 10, the sampling clock signal according to FIG. 9 is applied to the connection “T” of the flip-flop 32. The output connections of the flip-flop 32 are connected to a multiplicity of logical OR gates 33, 34, 35 and 36, which in turn supply signals to OR gates 37 and 38. The other input signals of the logic elements are provided by the device according to FIG. 7 and are given the same letters as in F i g. 7 and 8 designated. A negation is indicated in the usual way by a point at a connection of a logical element. Assuming that the output terminal Q of the flip-flop 32 initially has the state "0", the flip-flop 32 changes its state every time the applied sampling clock signal falls to "0". The output signals appearing on lines A, B and C for the two states of flip-flop 32 are then the following:

State 1

45

F i g. 6 shows the switching waveforms which are used in the circuit of FIG. 5 used transistors Qi, Q ₂ , etc. for the two phases of the in Fi g. 4A and 4B require the modulation signal shown. A signal greater than 0 indicates that the respective transistor is saturated

The absence of a signal means that the transistor is off. The circuit shown in FIG Fig. 7 creates the desired switching curve shapes. The illustrated flip-flops F / F are of a conventional type. The input signal supplied only has to have a state Be a pulse train with the correct pulse period, d. H.

= OQ = 1,

A = (fc +0)

B = A = k,

Note that k, k, ± h. / and e represent the group of switching signals required for phase 1 (see FIGS. 6 and 8).

γ. The various waveforms which are available at the signal points (a), (b), (c) ... of the circuit according to FIG. 7 are shown in FIG. 8, it is assumed that the output terminal Q of all flip-flops initially has the logical value "0", and that all Q = 1,0 = 0,

Note also that i. i, dhy 'and c / is the group of switching signals required for phase 2. If the output line B is applied to the transistor Q ₁ , the output line A to the transistor Qj and the output line C to the transistor Qi and Q * (Fig. 5), the desired multiplex combination of the two modules is achieved.

lator phases.

With each rising edge of the in FIG. 9 excites the monostable multivibrator 39 the sampler 41 (FIG. 5), which has a sampled representation Mu, / V / 2 ,, Afi, ... des at the output of the modulator appearing multiplex signal generated.

liier / u 4 matt drawings

Claims (2)

Patent claims: 1. N-path filter, formed from a plurality of input modulators for modulating a train-guided input signal, a filter arrangement connected to the outputs of the modulators for the modulated input signal and a plurality of output demodulators connected to the output of the filter arrangement for demodulation, ₀ des filtered signal, characterized in that the filter arrangement includes a sampler multiplexer (17) for forming multiplex sampling pulses of the modulated input signal, a single digital filter circuit (19) for filtering the multiplex sampling pulses and a demultiplexer circuit (21) for demultiplexing the filtered Signal from the digital filter circuit 2. N-path filter according to claim 1, characterized by the insertion of an analog-digital converter (18) between the sampler multiplexer (17) and digital filter (19), which encodes the multiplex sampling pulses from the sampler multiplexer (17) and a digital-to-analog converter (22) between the demultiplexer (21) and the output demodulator, which encodes the signal pulses from the demultiplexer (21) and its output is optionally connected to the demodulator.